1. Overview of Respiratory System Function

Gas Exchange and Respiratory Processes

The respiratory system is responsible for the exchange of gases between the atmosphere and the body, facilitating oxygen uptake and carbon dioxide removal. Gas exchange occurs at multiple levels, including between the lungs and blood, and between blood and tissues.

• External respiration: Exchange between alveoli and blood (pulmonary capillaries).

• Internal respiration: Exchange between blood (systemic capillaries) and cells.

• O2 diffusion: Into cells; CO2 into blood.

2. Histology of the Respiratory Tract

Cellular Structure and Function

The respiratory tract is lined with pseudostratified ciliated columnar epithelium containing goblet cells that secrete mucus for trapping particles. The mucosa transitions to simple cuboidal and then to simple squamous epithelium in the alveoli, allowing for efficient gas exchange.

• Cilia: Move trapped particles toward the pharynx to be swallowed or expectorated.

• Cartilage: Maintains airway structure; replaced by smooth muscle in smaller airways.

3. Anatomical Structures of the Upper Respiratory System

a. Pharynx

The pharynx is a muscular tube that serves as a passageway for air and food, extending from the base of the skull to the level of the cricoid cartilage.

• Nasopharynx: Posterior to the nasal cavity; lined with pseudostratified ciliated columnar epithelium.

• Oropharynx: Posterior to the oral cavity; lined with stratified squamous epithelium.

• Laryngopharynx: Opens into the larynx and esophagus; lined with stratified squamous epithelium.

b. Larynx

The larynx is the voice box, located between the pharynx and trachea. It contains several cartilages and is involved in sound production and airway protection.

• Thyroid cartilage: Largest cartilage; forms the Adam's apple.

• Cricoid cartilage: Forms the inferior wall of the larynx.

• Epiglottis: Covers the glottis during swallowing to prevent aspiration.

• Vocal folds: True vocal cords; vibrate to produce sound.

• Vestibular folds: False vocal cords; do not produce sound.

4. Anatomical Structures of the Lower Respiratory System

a. Trachea and Bronchial Tree

The trachea divides into right and left primary bronchi, which further branch into secondary and tertiary bronchi, and finally into bronchioles and terminal bronchioles.

• Carina: Ridge at the bifurcation of the trachea; sensitive area for cough reflex.

• Bronchi: Contain cartilage and smooth muscle; branch into smaller airways.

• Bronchioles: Lack cartilage; surrounded by smooth muscle; lead to alveoli.

5. The Respiratory Zone

Alveoli and Gas Exchange

The respiratory zone includes alveolar ducts, alveolar sacs, and alveoli, which are the primary sites of gas exchange.

• Type I alveolar cells: Form the gas exchange surface.

• Type II alveolar cells: Secrete surfactant to reduce surface tension and prevent alveolar collapse.

• Alveolar macrophages: Remove inhaled particles and pathogens.

6. Lungs

Lobes and Pleura

The lungs are divided into lobes and are surrounded by a double-layered pleural membrane.

• Right lung: Three lobes (superior, middle, inferior).

• Left lung: Two lobes (superior, inferior); contains cardiac notch.

• Pleura: Visceral pleura covers the lungs; parietal pleura lines the thoracic cavity.

• Pleural cavity: Contains pleural fluid to reduce friction and prevent lung collapse.

7. Mechanics of Breathing

Inspiration and Expiration

Breathing involves changes in thoracic volume and pressure, allowing air to flow into and out of the lungs.

• Inspiration: Diaphragm contracts and external intercostals elevate the ribs, increasing thoracic volume and decreasing intrapulmonary pressure ().

• Expiration: Usually passive; diaphragm and intercostals relax, decreasing thoracic volume and increasing intrapulmonary pressure ().

Muscles of forced inspiration: Sternocleidomastoid, scalenes, pectoralis minor.

Muscles of forced expiration: Internal intercostals, abdominal muscles.

8. Lungs Prevented from Collapsing

Pleural Adhesion and Pressure

• Pleural fluid: Creates surface tension that adheres the lungs to the thoracic wall.

• Transpulmonary pressure: Difference between alveolar and pleural pressure; keeps lungs inflated.

9. Lung Volumes and Capacities

Definitions and Table

Lung volumes and capacities are measured to assess respiratory function. The following table summarizes the main volumes and capacities:

10. Pulmonary Function Tests (PFTs)

Assessment of Lung Function

PFTs measure lung volumes and flow rates to diagnose respiratory diseases. Forced Vital Capacity (FVC) and Forced Expiratory Volume in 1 second (FEV1) are commonly used.

• Obstructive diseases: (e.g., asthma, COPD) show reduced FEV1/FVC ratio.

• Restrictive diseases: (e.g., pulmonary fibrosis) show reduced TLC and VC.

11. Minute and Alveolar Ventilation

Ventilation Calculations

• Minute ventilation:

• Alveolar ventilation:

Example: If tidal volume is 500 mL, dead space is 150 mL, and respiratory rate is 12 breaths/min:

• mL/min

• mL/min

12. Airway Resistance and Compliance

Factors Affecting Breathing

• Airway resistance: Controlled by airway diameter; bronchoconstriction increases resistance.

• Lung compliance: Measure of lung expandability; decreased in fibrosis, increased in emphysema.

Compliance equation:

13. Determinants of Compliance

Surface Tension, Elasticity, and Water Content

• Surface tension: Surfactant reduces surface tension, preventing alveolar collapse.

• Elastic fibers: Provide stretch and recoil.

• Water content: Excess fluid decreases compliance.

14. Gas Laws and Gas Exchange

Physical Principles of Gas Movement

• Dalton's Law: Total pressure of a gas mixture equals the sum of partial pressures of individual gases.

• Partial pressure calculation: mmHg

• Boyle's Law: At constant temperature, gas pressure is inversely proportional to volume:

• Charles' Law: Volume of a gas is directly proportional to absolute temperature (if pressure is constant).

15. Factors Influencing Alveolar Ventilation

Ventilation-Perfusion Matching

Alveolar ventilation depends on both the mechanics of airflow and the compliance of the lungs and chest wall. Two major factors are airway resistance and lung compliance.

• High compliance: Lungs expand easily; seen in emphysema.

• Low compliance: Lungs resist expansion; seen in fibrosis and pulmonary edema.

16. Clinical Examples

Respiratory Pathologies

• Emphysema: Destruction of elastic tissue increases compliance but decreases recoil.

• Pulmonary fibrosis: Collagen deposition decreases compliance.

• Asthma: Bronchoconstriction increases airway resistance.

• Respiratory acidosis: Accumulation of CO2 shifts the oxygen dissociation curve rightward.

17. Summary Table: Lung Volumes and Capacities

Additional info:

• Some details inferred from standard anatomy and physiology textbooks to clarify fragmented points.

• Clinical examples and equations added for academic completeness.